Printing systems that deflect drops using a gas flow are known; see, for example, U.S. Pat. No. 4,068,241, issued to Yamada, on Jan. 10, 1978. Such printing systems rely on the ability to generate distinct sizes of drop—a “print drop” of a given size, and a “catch drop” of distinctly different size. Differential deflection of the drops of different sizes is employed to cause print drops to impinge on the substrate and the catch drops to be collected and re-circulated through the ink delivery system.
In thermally stimulated continuous inkjet printing (see, for example Jeanmaire et al. U.S. Patent Application Publication No. 20020085071 A1 and Chwalek et al, In U.S. Pat. No. 6,079,821), periodic heat pulses are applied to individual heaters embedded in a nozzle array. The periodic heat pulses drive capillary break-up of jets formed at each nozzle to produce an array of drops. The period of the pulse waveform determines the ultimate size of drop formed after jet break-up. Because the jet responds most sensitively to disturbances at a characteristic frequency fR known as the Rayleigh frequency, drops are most effectively produced at a fundamental size corresponding to a volume of fluid given by πr2U/fR, where r is the jet radius and U is the jet velocity.
In U.S. Pat. No. 6,851,796, which issued on Feb. 8, 2005, an ink drop forming mechanism selectively creates a stream of ink drops having a plurality of different volumes traveling along a first path. An air flow directed across the stream of ink drops interacts with the stream of ink drops. This interaction deflects smaller drops more than larger drops and thereby separates ink drops having one volume from ink drops having other volumes.
As the drop selection mechanism described above depends on drop size, it is necessary for large-volume drops to be fully formed before being exposed to the deflection air flow. Consider, for example, a case where the large-volume drop is to have a volume equal to four small-volume drops. It is often seen during drop formation that the portion of the ink stream that is to form the large-volume drop will separate from the main stream as desired, but will then break apart before coalescing to form the large-volume drop. It is necessary for this coalescence to be complete prior to passing through the drop deflecting air flow. Otherwise the separate fragments that are to form the large-volume drop will be deflected by an amount greater than that of a single large-volume drop. Similarly, the small-volume drops must not merge in air before having past the deflection air flow. If separate small-volume drops merge, they will be deflected less than desired.
The distance over which the large-volume drop forms upon coalescence of is fragments is known as the drop formation length (DFL), denoted herein as LD. The details of the large-drop waveform and the physical properties of the jet determine the size of LD. For the purposes of printing, smaller drop formation lengths are advantageous, as the drops are then available for size separation at distances closer to the nozzle plate, and the distance over which the drops must travel prior to separation is reduced. Thus a smaller drop formation length helps reduce the size of the printhead and reduces the risk of incomplete large drop formation and reduces the risk of unintended merging of small drops.
It has been found that ink coverage levels are excessive when printing on certain print media, resulting loss of acuity and discernable gray levels. While the ink coverage level can be reduced through the use of smaller nozzles or by reducing the ink pressure or increasing the frequency of drop formation, these options have shortcomings. Conversely, on other substrates the ink coverage levels can be insufficient, resulting in lack of optical density and voids in the printed regions. While the ink coverage level can be altered through the use of different nozzles sizes or by adjusting the ink pressure or the frequency of drop formation, these options can also have shortcomings. If different nozzle sizes are to be used for different print media, then it would be necessary to produce and maintain an inventory of a number of distinct printheads each having a distinct nozzle size. Reducing the ink pressure or raising the frequency of drop formation can result in reducing the stimulation perturbation wavelengths toward the Rayleigh cutoff limit. As the perturbation wavelengths are reduced toward the Rayleigh cutoff limit, the drop formation can become excessively sensitive to small changes in ink properties, nozzle size, ink pressure, and stimulation amplitude. Increasing the ink pressure or reducing the frequency, on the other hand, can increase the formation of satellite drops, which can reduce printhead reliability.
Thus there is a need for waveforms that provide a means to alter the size of the large drops relative to the small drops. The present invention addresses these needs.